The principal aim is to examine ion and molecule transport into bone and efflux of ions and molecules from bone. The major techniques will be indicator dilution techniques and will emphasize determination of extraction after a single transcapillary passage and subsequent washout of ions and molecules from bone and their rate of emergence, that is fractional escape rate (FER). The role of bone in moment-to-moment calcium ion homeostasis will be evaluated by these methods in normal bone and aging canine bone. The significance of these studies is to better understand mechanisms of bone loss and the eventual osteopenia which is a major cause of fracture in the elderly. A second aim is to examine the effect of external fixators on healing of an experimental ribial fracture. Blood flow will be evaluated by microspheres and compared to deposition of scanning agents, diphosphonates. This comparison, blood flow determined by microspheres to deposition of (99mTc)-diphosphonates, will be performed in dogs that are mature, adolescent, growing and with augmented and decreased blood flow. If a close correlation exists between deposition of the scanning can be used as an index of blood flow in bone. Speed of bone healing will be measured by ciomechanical testing (torsional testing). The significance of this study is to allow a systematic evaluation of external fixation as a method of fracture fixation.